Illumination Source for an Inspection Apparatus, Inspection Apparatus and Inspection Method

ABSTRACT

An illumination source apparatus ( 500 ), suitable for use in a metrology apparatus for the characterization of a structure on a substrate, the illumination source apparatus comprising: a high harmonic generation, HHG, medium ( 502 ); a pump radiation source ( 506 ) operable to emit a beam of pump radiation ( 508 ); and adjustable transformation optics ( 510 ) configured to adjustably transform the transverse spatial profile of the beam of pump radiation to produce a transformed beam ( 518 ) such that relative to the centre axis of the transformed beam, a central region of the transformed beam has substantially zero intensity and an outer region which is radially outwards from the centre axis of the transformed beam has a non-zero intensity, wherein the transformed beam is arranged to excite the HHG medium so as to generate high harmonic radiation ( 540 ), wherein the location of said outer region is dependent on an adjustment N setting of the adjustable transformation optics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of EP application 18172113.5 which wasfiled on 14 May 2018 and which is incorporated herein in its entirety byreference.

TECHNICAL FIELD

The present invention relates to an inspection apparatus and a methodfor performing a measurement. In particular, it relates to anillumination source apparatus suitable for use in a metrology apparatusfor the characterization of a structure on a substrate.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may, for example, project a pattern (also often referred to as“design layout” or “design”) at a patterning device (e.g., a mask) ontoa layer of radiation-sensitive material (resist) provided on a substrate(e.g., a wafer).

To project a pattern on a substrate a lithographic apparatus may useelectromagnetic radiation. The wavelength of this radiation determinesthe minimum size of features which can be formed on the substrate.Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nmand 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet(EUV) radiation, having a wavelength within the range 4-20 nm, forexample 6.7 nm or 13.5 nm, may be used to form smaller features on asubstrate than a lithographic apparatus which uses, for example,radiation with a wavelength of 193 nm.

Low-k₁ lithography may be used to process features with dimensionssmaller than the classical resolution limit of a lithographic apparatus.In such a process, the resolution formula may be expressed asCD=k₁×λ/NA, where λ is the wavelength of radiation employed, NA is thenumerical aperture of the projection optics in the lithographicapparatus, CD is the “critical dimension” (generally the smallestfeature size printed, but in this case half-pitch) and k₁ is anempirical resolution factor. In general, the smaller k₁ the moredifficult it becomes to reproduce the pattern on the substrate thatresembles the shape and dimensions planned by a circuit designer inorder to achieve particular electrical functionality and performance. Toovercome these difficulties, sophisticated fine-tuning steps may beapplied to the lithographic projection apparatus and/or design layout.These include, for example, but not limited to, optimization of NA,customized illumination schemes, use of phase shifting patterningdevices, various optimization of the design layout such as opticalproximity correction (OPC, sometimes also referred to as “optical andprocess correction”) in the design layout, or other methods generallydefined as “resolution enhancement techniques” (RET). Alternatively,tight control loops for controlling a stability of the lithographicapparatus may be used to improve reproduction of the pattern at low k₁.

In lithographic processes, it is desirable to frequently makemeasurements of the structures created, e.g., for process control andverification. Tools to make such measurements are typically calledmetrology tools MT. Different types of metrology tools MT for makingsuch measurements are known, including scanning electron microscopes orvarious forms of scatterometer metrology tools MT.

As an alternative to optical metrology methods, it has also beenconsidered to use soft X-rays and/or EUV radiation, for exampleradiation in a wavelength range between 0.1 nm and 100 nm, or optionallybetween 1 nm and 50 nm or optionally between 10 nm and 20 nm. A sourcefor generating the soft X-rays and/or EUV radiation may be a source thatuses the principle of High Harmonic Generation (HHG).

A problem addressed by the current invention is how to improve theoutput power of a high harmonic generation, HHG, illumination sourceused to produce soft x-ray and/or EUV radiation for use in a metrologytool.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedan illumination source apparatus, suitable for use in a metrologyapparatus for the characterization of a structure on a substrate, theillumination source apparatus comprising: a high harmonic generation,HHG, medium; a pump radiation source operable to emit a beam of pumpradiation; and adjustable transformation optics configured to adjustablytransform the transverse spatial profile of the beam of pump radiationto produce a transformed beam such that relative to the centre axis ofthe transformed beam, a central region of the transformed beam hassubstantially zero intensity and an outer region which is radiallyoutwards from the centre axis of the transformed beam has a non-zerointensity, wherein the transformed beam is arranged to excite the HHGmedium so as to generate high harmonic radiation, wherein the locationof said outer region is dependent on an adjustment setting of theadjustable transformation optics.

The pump radiation source may be operable to emit a beam of pumpradiation with a Gaussian transverse spatial profile, and the adjustabletransformation optics may be configured to produce a transformed beamwith a non-Gaussian or annular transverse spatial profile.

The illumination source apparatus may further comprise a focusingelement positioned between the adjustable transformation optics and theHHG medium, the focusing element configured to focus the transformedbeam into the HHG medium. The focusing element may be a lens. The focalplane of the focusing element may be positioned substantially in the HHGmedium.

The adjustable transformation optics may comprise at least one conicalelement such as an axicon element.

For example, the adjustable transformation optics may comprise a pair ofaxicon elements consisting of a first axicon element and a second axiconelement, wherein the first axicon element precedes the second axiconelement relative to the propagation direction of the beam of pumpradiation and wherein an axial separation between the first axiconelement and the second axicon element controls said adjustment settingof the adjustable transformation optics.

The pair of axicon elements may consist of one negative axicon elementand one positive axicon element. The axicon elements may be reflectiveaxicon elements, refractive axicon elements, or diffractive axiconelements or the pair may be a combination of different types of axiconelements.

The first axicon element of the pair may be a negative reflective axiconwhich is arranged on the centre axis of the beam of pump radiation andwhich is configured to reflect the beam of pump radiation towards thesecond axicon element which may be an annular positive reflective axiconconfigured to collimate the beam to thereby produce said transformedbeam.

The first axicon element of the pair may alternatively be a negativerefractive axicon which is arranged on the centre axis of the beam ofpump radiation and configured to diverge the beam of pump radiationtowards the second axicon element which may be a positive refractiveaxicon arranged on said centre axis and configured to collimate the beamto thereby produce said transformed beam.

The first axicon element of the pair may alternatively be a negativediffractive axicon which is arranged on the centre axis of the beam ofpump radiation and configured to diverge the beam of pump radiationtowards the second axicon element which may be a positive diffractiveaxicon arranged on said centre axis and configured to collimate the beamto thereby produce said transformed beam.

For reflective axicon elements and refractive axicon elements, eachaxicon of the pair of axicon elements may have substantially the sameapex angle, τ. The diffractive optical elements (DOE) axicon is definednot by an apex angle τ but a divergence angle, β. The divergence angle βis equivalent to twice as the deflection angle (2*Y). For diffractiveaxicon elements, each axicon of the pair of axicon elements may havesubstantially the same divergence angle β. The axicon elements may bemounted on one or more movable mounts such that said axial separation,D₁, between the axicon elements is adjustable in use to control saidadjustment setting.

The illumination source apparatus may further comprise a blockingelement positioned after the HHG medium, the blocking element configuredto suppress the residual transformed beam remaining after high harmonicgeneration, whilst substantially transmitting the generated highharmonic radiation.

The blocking element may be an output aperture aligned with the centreaxis of the generated high harmonic radiation.

The focusing element may be configured to image the first axicon elementonto the blocking element.

During use of the illumination source apparatus, the axial separation ofthe axicons, D₁, may be selected by means of the one or more movablemounts in order to optimize, for a given axicon apex angle τ or for agiven axicon divergence angle β:

-   -   (A) the conversion efficiency of the high harmonic generation        process; and/or    -   (B) the suppression of the residual transformed beam.

The adjustable transformation optics may further comprise a variablebeam expander/contractor configured to adjust the input waist size, w₀,of the beam of pump radiation, and wherein in use w₀ is selected inorder to further optimize (A) and (B).

The transformed beam may be a collimated annular beam having an annulusradius R₁ and a ring width R₂ given by:

R ₁ =D ₁ tan(γ); and

R ₂ =R ₁ +w ₀,

where the deflection angle γ is given by equations (1) or (5) and (7) or(8) below in the detailed description, or is otherwise related to theaxicon apex angle τ or axicon divergence angle β.

The adjustable transformation optics may further comprise an inputaperture located on the centre axis of the beam of pump radiation,wherein the input aperture is positioned after the axicon elements andprior to the HHG medium with respect to the direction of propagation ofthe beam of pump radiation, and the focusing element is configured toimage the input aperture onto the blocking element, and wherein in usethe aperture size of the input aperture is selected in order to furtheroptimize (A) and (B). The input aperture may be configured to adjust thering width R₂.

According to a second aspect of the present invention there is provideda method of operating an illumination source apparatus, suitable for usein a metrology apparatus for the characterization of a structure on asubstrate, the method comprising: providing a high harmonic generation,HHG, medium; operating a pump radiation source to emit a beam of pumpradiation; and transforming, by adjustable transformation optics, thetransverse spatial profile of the beam of pump radiation to produce atransformed beam such that relative to the centre axis of thetransformed beam, a central region of the transformed beam hassubstantially zero intensity and an outer region which is radiallyoutwards from the centre axis of the transformed beam has a non-zerointensity, wherein the transformed beam excites the HHG medium so as togenerate high harmonic radiation, wherein the location of said outerregion is dependent on an adjustment setting of the adjustabletransformation optics.

The pump radiation source may emit a beam of pump radiation with aGaussian transverse spatial profile, and the adjustable transformationoptics may produce a transformed beam with a non-Gaussian or annulartransverse spatial profile.

The method according to the second aspect of the present invention mayfurther comprise focusing the transformed beam into the HHG medium usinga focusing element positioned between the adjustable transformationoptics and the HHG medium.

The adjustable transformation optics may comprise a pair of axiconelements consisting of a first axicon element and a second axiconelement, wherein the first axicon element precedes the second axiconelement relative to the propagation direction of the beam of pumpradiation and wherein the axial separation between the first axiconelement and the second axicon element is adjusted to control saidadjustment setting of the adjustable transformation optics.

For reflective axicon elements and refractive axicon elements, each achaxicon of the pair of axicon elements may have substantially the sameapex angle, τ. The diffractive optical elements (DOE) axicon is definednot by an apex angle τ but a divergence angle, β. For diffractive axiconelements, each axicon of the pair of axicon elements may havesubstantially the same divergence angle, β. The axicon elements may bemounted on one or more movable mounts, and the method may furthercomprise adjusting the axial separation, D₁, between the axicon elementsto control said adjustment setting.

The method according to the second aspect may further comprisesuppressing, using a blocking element positioned after the HHG medium,the residual transformed beam remaining after high harmonic generation,whilst substantially transmitting the generated high harmonic radiation.The first axicon element may be imaged onto the blocking element usingthe focusing element.

The method may further comprise selecting the axial separation, D₁,between the axicon elements in order to optimize:

-   -   (A) the conversion efficiency of the high harmonic generation        process; and/or    -   (B) the suppression of the residual transformed beam.

The method may further comprise adjusting the input waist size, w₀, ofthe beam of pump radiation using a variable beam expander/contractor, inorder to further optimize (A) and (B).

The adjustable transformation optics may further comprise an input irislocated on the centre axis of the beam of pump radiation, wherein theinput iris is positioned after the axicon elements and prior to the HHGmedium with respect to the direction of propagation of the beam of pumpradiation, and the focusing element is configured to image the inputiris onto the blocking element, and wherein the aperture size of theinput iris is selected in order to further optimize (A) and (B).

According to a third aspect of the present invention there is provided acomputer program comprising instructions which, when executed on atleast one processor, cause the at least one processor to control anapparatus to carry out a method according to the second aspect of theinvention.

According to a fourth aspect of the present invention there is provideda carrier containing the computer program according to third aspect,wherein the carrier is one of an electronic signal, optical signal,radio signal, or non-transitory computer readable storage medium.

According to a fifth aspect of the present invention there is provided alithographic apparatus comprising the illumination source apparatusaccording to the first aspect of the invention.

According to a sixth aspect of the invention there is provided alithographic cell comprising the lithographic apparatus according to thefifth aspect of the invention.

According to a seventh aspect of the present invention there is provideda metrology apparatus comprising an illumination source apparatusaccording to the first aspect of the invention.

According to an eighth aspect of the present invention there is provideda lithographic cell comprising a metrology apparatus according to theseventh aspect of the invention.

The present invention aims to provide improved output power of a highharmonic generation, HHG, illumination source apparatus used to producesoft X-ray and/or EUV radiation for use in a metrology tool. Theinvention achieves this through enabling improved conversion efficiencyand reduced reliance on metal filters for suppressing the residual pumpradiation.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 depicts a schematic overview of a lithographic apparatus;

FIG. 2 depicts a schematic overview of a lithographic cell;

FIG. 3 depicts a schematic representation of holistic lithography,representing a cooperation between three key technologies to optimizesemiconductor manufacturing;

FIG. 4 depicts a schematic representation of a metrology apparatus inwhich radiation in the wavelength range from 0.1 nm to 100 nm may beused to measure parameters/characteristics of structures on a substrate;

FIG. 5 depicts an illumination source apparatus according to a firstembodiment of the invention;

FIG. 6 depicts a sample transverse intensity profile in the (x, y) planefor a transformed beam produced in the illumination source apparatus;

FIG. 7 depicts an illumination source apparatus according to a secondembodiment of the invention;

FIG. 8 depicts an illumination source apparatus according to a thirdembodiment of the invention;

FIG. 9 depicts the transformation optics of an illumination sourceapparatus according to a fourth embodiment of the invention;

FIGS. 10a to 10c depict axicon elements used in the fourth embodiment;

FIG. 11 shows the results of numerical modeling of the phase velocitythrough the focus of various annular beams of increasing inner radius;

FIG. 12 shows results of numerical modeling of the suppression of theresidual transformed beam as a function of the input pump beam waist w₀and the axicon pair separation D₁;

FIG. 13 is a flowchart of a method according to an embodiment of theinvention;

FIGS. 14a and 14b depict negative and positive refractive axiconelements and their influence on an incident beam of radiation; and

FIG. 15 depicts the transformation optics of an illumination sourceapparatus according to a fifth embodiment of the invention.

FIG. 16 depicts an illumination source apparatus according to a sixthembodiment of the invention;

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Before describing embodiments of the invention in detail, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm), EUV(extreme ultra-violet radiation, e.g. having a wavelength in the rangeof about 1-100 nm) and/or soft X-ray radiation (e.g. radiation in awavelength range from 0.1 to 10 nm).

The term “reticle”, “mask” or “patterning device” as employed in thistext may be broadly interpreted as referring to a generic patterningdevice that can be used to endow an incoming radiation beam with apatterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate. The term “light valve” canalso be used in this context. Besides the classic mask (transmissive orreflective, binary, phase-shifting, hybrid, etc.), examples of othersuch patterning devices include a programmable mirror array and aprogrammable LCD array.

FIG. 1 schematically depicts a lithographic apparatus LA. Thelithographic apparatus LA includes an illumination system (also referredto as illuminator) IL configured to condition a radiation beam B (e.g.,UV radiation, DUV radiation or EUV radiation), a mask support (e.g., amask table) MT constructed to support a patterning device (e.g., a mask)MA and connected to a first positioner PM configured to accuratelyposition the patterning device MA in accordance with certain parameters,a substrate support (e.g., a wafer table) WT constructed to hold asubstrate (e.g., a resist coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate support inaccordance with certain parameters, and a projection system (e.g., arefractive projection lens system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g., comprising one or more dies) of the substrate W.

In operation, the illumination system IL receives a radiation beam froma radiation source SO, e.g. via a beam delivery system BD. Theillumination system IL may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic,electrostatic, and/or other types of optical components, or anycombination thereof, for directing, shaping, and/or controllingradiation. The illuminator IL may be used to condition the radiationbeam B to have a desired spatial and angular intensity distribution inits cross section at a plane of the patterning device MA.

The term “projection system” PS used herein should be broadlyinterpreted as encompassing various types of projection system,including refractive, reflective, catadioptric, anamorphic, magnetic,electromagnetic and/or electrostatic optical systems, or any combinationthereof, as appropriate for the exposure radiation being used, and/orfor other factors such as the use of an immersion liquid or the use of avacuum. Any use of the term “projection lens” herein may be consideredas synonymous with the more general term “projection system” PS.

The lithographic apparatus LA may be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system PS and the substrate W—which is also referred to asimmersion lithography. More information on immersion techniques is givenin U.S. Pat. No. 6,952,253, which is incorporated herein by reference.

The lithographic apparatus LA may also be of a type having two or moresubstrate supports WT (also named “dual stage”). In such “multiplestage” machine, the substrate supports WT may be used in parallel,and/or steps in preparation of a subsequent exposure of the substrate Wmay be carried out on the substrate W located on one of the substratesupport WT while another substrate W on the other substrate support WTis being used for exposing a pattern on the other substrate W.

In addition to the substrate support WT, the lithographic apparatus LAmay comprise a measurement stage. The measurement stage is arranged tohold a sensor and/or a cleaning device. The sensor may be arranged tomeasure a property of the projection system PS or a property of theradiation beam B. The measurement stage may hold multiple sensors. Thecleaning device may be arranged to clean part of the lithographicapparatus, for example a part of the projection system PS or a part of asystem that provides the immersion liquid. The measurement stage maymove beneath the projection system PS when the substrate support WT isaway from the projection system PS.

In operation, the radiation beam B is incident on the patterning device,e.g. mask, MA which is held on the mask support MT, and is patterned bythe pattern (design layout) present on patterning device MA. Havingtraversed the mask MA, the radiation beam B passes through theprojection system PS, which focuses the beam onto a target portion C ofthe substrate W. With the aid of the second positioner PW and a positionmeasurement system IF, the substrate support WT can be moved accurately,e.g., so as to position different target portions C in the path of theradiation beam B at a focused and aligned position. Similarly, the firstpositioner PM and possibly another position sensor (which is notexplicitly depicted in FIG. 1) may be used to accurately position thepatterning device MA with respect to the path of the radiation beam B.Patterning device MA and substrate W may be aligned using mask alignmentmarks M1, M2 and substrate alignment marks P1, P2. Although thesubstrate alignment marks P1, P2 as illustrated occupy dedicated targetportions, they may be located in spaces between target portions.Substrate alignment marks P1, P2 are known as scribe-lane alignmentmarks when these are located between the target portions C.

As shown in FIG. 2 the lithographic apparatus LA may form part of alithographic cell LC, also sometimes referred to as a lithocell or(litho)cluster, which often also includes apparatus to perform pre- andpost-exposure processes on a substrate W. Conventionally these includespin coaters SC to deposit resist layers, developers DE to developexposed resist, chill plates CH and bake plates BK, e.g. forconditioning the temperature of substrates W e.g. for conditioningsolvents in the resist layers. A substrate handler, or robot, RO picksup substrates W from input/output ports I/O1, I/O2, moves them betweenthe different process apparatus and delivers the substrates W to theloading bay LB of the lithographic apparatus LA. The devices in thelithocell, which are often also collectively referred to as the track,are typically under the control of a track control unit TCU that initself may be controlled by a supervisory control system SCS, which mayalso control the lithographic apparatus LA, e.g. via lithography controlunit LACU.

In order for the substrates W exposed by the lithographic apparatus LAto be exposed correctly and consistently, it is desirable to inspectsubstrates to measure properties of patterned structures, such asoverlay errors between subsequent layers, line thicknesses, criticaldimensions (CD), etc. For this purpose, inspection tools (not shown) maybe included in the lithocell LC. If errors are detected, adjustments,for example, may be made to exposures of subsequent substrates or toother processing steps that are to be performed on the substrates W,especially if the inspection is done before other substrates W of thesame batch or lot are still to be exposed or processed.

An inspection apparatus, which may also be referred to as a metrologyapparatus, is used to determine properties of the substrates W, and inparticular, how properties of different substrates W vary or howproperties associated with different layers of the same substrate W varyfrom layer to layer. The inspection apparatus may alternatively beconstructed to identify defects on the substrate W and may, for example,be part of the lithocell LC, or may be integrated into the lithographicapparatus LA, or may even be a stand-alone device. The inspectionapparatus may measure the properties on a latent image (image in aresist layer after the exposure), or on a semi-latent image (image in aresist layer after a post-exposure bake step PEB), or on a developedresist image (in which the exposed or unexposed parts of the resist havebeen removed), or even on an etched image (after a pattern transfer stepsuch as etching).

Typically the patterning process in a lithographic apparatus LA is oneof the most critical steps in the processing which requires highaccuracy of dimensioning and placement of structures on the substrate W.To ensure this high accuracy, three systems may be combined in a socalled “holistic” control environment as schematically depicted in FIG.3. One of these systems is the lithographic apparatus LA which is(virtually) connected to a metrology tool MT (a second system) and to acomputer system CL (a third system). The key of such “holistic”environment is to optimize the cooperation between these three systemsto enhance the overall process window and provide tight control loops toensure that the patterning performed by the lithographic apparatus LAstays within a process window. The process window defines a range ofprocess parameters (e.g. dose, focus, overlay) within which a specificmanufacturing process yields a defined result (e.g. a functionalsemiconductor device)—typically within which the process parameters inthe lithographic process or patterning process are allowed to vary.

The computer system CL may use (part of) the design layout to bepatterned to predict which resolution enhancement techniques to use andto perform computational lithography simulations and calculations todetermine which mask layout and lithographic apparatus settings achievethe largest overall process window of the patterning process (depictedin FIG. 3 by the double arrow in the first scale SC1). Typically, theresolution enhancement techniques are arranged to match the patterningpossibilities of the lithographic apparatus LA. The computer system CLmay also be used to detect where within the process window thelithographic apparatus LA is currently operating (e.g. using input fromthe metrology tool MT) to predict whether defects may be present due toe.g. sub-optimal processing (depicted in FIG. 3 by the arrow pointing“0” in the second scale SC2).

The metrology tool MT may provide input to the computer system CL toenable accurate simulations and predictions, and may provide feedback tothe lithographic apparatus LA to identify possible drifts, e.g. in acalibration status of the lithographic apparatus LA (depicted in FIG. 3by the multiple arrows in the third scale SC3).

In lithographic processes, it is desirable to frequently makemeasurements of the structures created, e.g., for process control andverification. Tools to make such measurements are typically calledmetrology tools MT. Different types of metrology tools MT for makingsuch measurements are known, including scanning electron microscopes orvarious forms of scatterometer metrology tools MT.

Scatterometers are versatile instruments which allow measurements of theparameters of a lithographic process by having a sensor in the pupil ora conjugate plane with the pupil of the objective of the scatterometer,measurements usually referred to as pupil based measurements, or byhaving the sensor in the image plane or a plane conjugate with the imageplane, in which case the measurements are usually referred to as imageor field based measurements. Such scatterometers and the associatedmeasurement techniques are further described in patent applicationsUS20100328655, US2011102753A1, US20120044470A, US20110249244,US20110026032 or EP1,628,164A, incorporated herein by reference in theirentirety. Aforementioned scatterometers may measure gratings using lightfrom soft x-ray, Extreme Ultraviolet (EUV) and visible to near-IRwavelength range.

A first type of scatterometer is an angular resolved scatterometer. Insuch a scatterometer reconstruction methods may be applied to themeasured signal to reconstruct or calculate properties of the grating.Such reconstruction may, for example, result from simulating interactionof scattered radiation with a mathematical model of the target structureand comparing the simulation results with those of a measurement.Parameters of the mathematical model are adjusted until the simulatedinteraction produces a diffraction pattern similar to that observed fromthe real target.

A second type of scatterometer is a spectroscopic scatterometer MT. Insuch spectroscopic scatterometer MT, the radiation emitted by aradiation source is directed onto the target and the reflected orscattered radiation from the target is directed to a spectrometerdetector, which measures a spectrum (i.e. a measurement of intensity asa function of wavelength) of the specular reflected radiation (i.e. the0th order). From this data, the structure or profile of the targetgiving rise to the detected spectrum may be reconstructed, e.g. byRigorous Coupled Wave Analysis and non-linear regression or bycomparison with a library of simulated spectra.

A third type of scatterometer is an ellipsometric scatterometer. Theellipsometric scatterometer allows for determining parameters of alithographic process by measuring scattered radiation for eachpolarization states. Such metrology apparatus emits polarized light(such as linear, circular, or elliptic) by using, for example,appropriate polarization filters in the illumination section of themetrology apparatus. A source suitable for the metrology apparatus mayprovide polarized radiation as well. Various embodiments of existingellipsometric scatterometers are described in U.S. patent applicationSer. Nos. 11/451,599, 11/708,678, 12/256,780, 12/486,449, 12/920,968,12/922,587, 13/000,229, 13/033,135, 13/533,110 and 13/891,410incorporated herein by reference in their entirety.

The scatterometer MT may be adapted to measure the overlay of twomisaligned gratings or periodic structures by measuring asymmetry in thereflected spectrum and/or the detection configuration, the asymmetrybeing related to the extent of the overlay. The two (typicallyoverlapping) grating structures may be applied in two different layers(not necessarily consecutive layers), and may be formed substantially atthe same position on the wafer. The scatterometer may have a symmetricaldetection configuration as described e.g. in patent applicationEP1,628,164A, the entire contents of which are incorporated herein byreference, such that any asymmetry is clearly distinguishable. Thisprovides a straightforward way to measure misalignment in gratings.Further examples for measuring overlay error between the two layerscontaining periodic structures as target through asymmetry of theperiodic structures may be found in PCT patent application publicationno. WO 2011/012624 or US patent application US 20160161863, incorporatedherein by reference in its entirety.

Other parameters of interest may be focus and dose and more inparticular the focus and dose being used by the lithographic apparatuswhile printing a pattern on a substrate. Focus and dose may bedetermined simultaneously by scatterometry (or alternatively by scanningelectron microscopy) as described in US patent applicationUS2011-0249244, incorporated herein by reference in its entirety. Asingle structure may be used which has a unique combination of criticaldimension and sidewall angle measurements for each point in a focusenergy matrix (FEM—also referred to as Focus Exposure Matrix). If theseunique combinations of critical dimension and sidewall angle areavailable, the focus and dose values may be uniquely determined fromthese measurements.

A metrology target may be an ensemble of composite gratings, formed by alithographic process, mostly in resist, but also after etch processingfor example. Typically the pitch and line-width of the structures in thegratings strongly depend on the measurement optics (in particular the NAof the optics) to be able to capture diffraction orders coming from themetrology targets. As indicated earlier, the diffracted signal may beused to determine shifts between two layers (also referred to ‘overlay’)or may be used to reconstruct at least part of the original grating asproduced by the lithographic process. This reconstruction may be used toprovide guidance of the quality of the lithographic process and may beused to control at least part of the lithographic process. Targets mayhave smaller sub-segmentation which are configured to mimic dimensionsof the functional part of the design layout in a target. Due to thissub-segmentation, the targets will behave more similar to the functionalpart of the design layout such that the overall process parametermeasurements resemble the functional part of the design layout better.The targets may be measured in an under-filled mode or in an overfilledmode. In the under-filled mode, the measurement beam generates a spotthat is smaller than the overall target. In the overfilled mode, themeasurement beam generates a spot that is larger than the overalltarget. In such overfilled mode, it may also be possible to measuredifferent targets simultaneously, thus determining different processingparameters at the same time.

Overall measurement quality of a lithographic parameter using a specifictarget is at least partially determined by the measurement recipe usedto measure this lithographic parameter. The term “substrate measurementrecipe” may include one or more parameters of the measurement itself,one or more parameters of the one or more patterns measured, or both.For example, if the measurement used in a substrate measurement recipeis a diffraction-based optical measurement, one or more of theparameters of the measurement may include the wavelength of theradiation, the polarization of the radiation, the incident angle ofradiation relative to the substrate, the orientation of radiationrelative to a pattern on the substrate, etc. One of the criteria toselect a measurement recipe may, for example, be a sensitivity of one ofthe measurement parameters to processing variations. More examples aredescribed in US patent application US2016-0161863 and not yet publishedU.S. patent application Ser. No. 15/181,126, incorporated herein byreference in its entirety.

As an alternative to optical metrology methods, it has also beenconsidered to use soft X-rays and/or EUV radiation, for exampleradiation in a wavelength range between 0.1 nm and 100 nm, or optionallybetween 1 nm and 50 nm or optionally between 10 nm and 20 nm. Oneexample of a metrology tool functioning in one of the above presentedwavelength ranges is transmissive small angle X-ray scattering (T-SAXSas in US 2007224518A which content is incorporated herein by referencein its entirety). Profile (CD) measurements using T-SAXS are discussedby Lemaillet et al in “Intercomparison between optical and X-rayscatterometry measurements of FinFET structures”, Proc. of SPIE, 2013,8681. Reflectometry techniques using X-rays (GI-XRS) and extremeultraviolet (EUV) radiation at grazing incidence are known for measuringproperties of films and stacks of layers on a substrate. Within thegeneral field of reflectometry, goniometric and/or spectroscopictechniques can be applied. In goniometry, the variation of a reflectedbeam with different incidence angles is measured. Spectroscopicreflectometry, on the other hand, measures the spectrum of wavelengthsreflected at a given angle (using broadband radiation). For example, EUVreflectometry has been used for inspection of mask blanks, prior tomanufacture of reticles (patterning devices) for use in EUV lithography.

It is possible that the range of application makes the use ofwavelengths in the soft X-rays and/or EUV domain not sufficient.Therefore published patent applications US 20130304424A1 andUS2014019097A1 (Bakeman et al/KLA) describe hybrid metrology techniquesin which measurements made using x-rays and optical measurements withwavelengths in the range 120 nm and 2000 nm are combined together toobtain a measurement of a parameter such as CD. A CD measurement isobtained by coupling and x-ray mathematical model and an opticalmathematical model through one or more common. The contents of the citedUS patent applications are incorporated herein by reference in theirentirety.

FIG. 4 depicts a schematic representation of a metrology apparatus 302in which radiation in the wavelength range from 0.1 nm to 100 nm may beused to measure parameters of structures on a substrate. The metrologyapparatus 302 presented in FIG. 4 is suitable for the soft X-rays and/orEUV domain.

FIG. 4 illustrates a schematic physical arrangement of a metrologyapparatus 302 comprising a spectroscopic scatterometer using EUV and/orSXR radiation in grazing incidence, purely by way of example. Analternative form of inspection apparatus might be provided in the formof an angle-resolved scatterometer, which uses radiation in normal ornear-normal incidence similar to the conventional scatterometersoperating at longer wavelengths.

Inspection apparatus 302 comprises a radiation source 310, illuminationsystem 312, substrate support 316, detection systems 318, 398 andmetrology processing unit (MPU) 320.

Source 310 in this example comprises a generator of EUV and/or softx-ray radiation based on high harmonic generation (HHG) techniques. Suchsources are available for example from KMLabs, Boulder Colo., USA(http://www.kmlabs.com/). Main components of the radiation source are adrive laser 330 and an HHG gas cell 332. A gas supply 334 suppliessuitable gas to the gas cell, where it is optionally ionized by anelectric source 336. The drive laser 300 may be, for example, afiber-based laser with an optical amplifier, producing pulses ofinfrared radiation that may last for example less than 1 ns (1nanosecond) per pulse, with a pulse repetition rate up to severalmegahertz, as required. The wavelength of the infrared radiation may befor example in the region of 1 (1 micron). The laser pulses aredelivered as a first radiation beam 340 to the HHG gas cell 332, wherein the gas a portion of the radiation is converted to higher frequenciesthan the first radiation into a beam 342 including coherent secondradiation of the desired wavelength or wavelengths.

The second radiation may contain multiple wavelengths. If the radiationwere monochromatic, then measurement calculations (for examplereconstruction) may be simplified, but it is easier with HHG to produceradiation with several wavelengths. The volume of gas within the gascell 332 defines an HHG space, although the space need not be completelyenclosed and a flow of gas may be used instead of a static volume. Thegas may be for example a noble gas such as neon (Ne) or argon (Ar). N₂,O₂, He, Ar, Kr, Xe gases can all be considered. These are matters ofdesign choice, and may even be selectable options within the sameapparatus. Different wavelengths will, for example, provide differentlevels of contrast when imaging structure of different materials. Forinspection of metal structures or silicon structures, for example,different wavelengths may be selected to those used for imaging featuresof (carbon-based) resist, or for detecting contamination of suchdifferent materials. One or more filtering devices 344 may be provided.For example a filter such as a thin membrane of Aluminum (Al) orZirconium (ZR) may serve to cut the fundamental IR radiation frompassing further into the inspection apparatus. A grating (not shown) maybe provided to select one or more specific harmonic wavelengths fromamong those generated in the gas cell. Some or all of the beam path maybe contained within a vacuum environment, bearing in mind that SXRradiation is absorbed when traveling in air. The various components ofradiation source 310 and illumination optics 312 can be adjustable toimplement different metrology ‘recipes’ within the same apparatus. Forexample different wavelengths and/or polarization can be madeselectable.

Depending on the materials of the structure under inspection, differentwavelengths may offer a desired level of penetration into lower layers.For resolving the smallest device features and defects among thesmallest device features, then a short wavelength is likely to bepreferred. For example, one or more wavelengths in the range 1-20 nm oroptionally in the range 1-10 nm or optionally in the range 10-20 nm maybe chosen. Wavelengths shorter than 5 nm suffer from very low criticalangle when reflecting off materials typically of interest insemiconductor manufacture. Therefore to choose a wavelength greater than5 nm will provide stronger signals at higher angles of incidence. On theother hand, if the inspection task is for detecting the presence of acertain material, for example to detect contamination, then wavelengthsup to 50 nm could be useful.

From the radiation source 310, the filtered beam 342 enters aninspection chamber 350 where the substrate W including a structure ofinterest is held for inspection at a measurement position by substratesupport 316. The structure of interest is labeled T. The atmospherewithin inspection chamber 350 is maintained near vacuum by vacuum pump352, so that EUV radiation can pass without undue attenuation throughthe atmosphere. The Illumination system 312 has the function of focusingthe radiation into a focused beam 356, and may comprise for example atwo-dimensionally curved mirror, or a series of one-dimensionally curvedmirrors, as described in published US patent applicationUS2017/0184981A1 (which content is incorporated herein by reference inits entirety), mentioned above. The focusing is performed to achieve around or elliptical spot S under 10 μm in diameter, when projected ontothe structure of interest. Substrate support 316 comprises for examplean X-Y translation stage and a rotation stage, by which any part of thesubstrate W can be brought to the focal point of beam to in a desiredorientation. Thus the radiation spot S is formed on the structure ofinterest. Alternatively, or additionally, substrate support 316comprises for example a tilting stage that may tilt the substrate W at acertain angle to control the angle of incidence of the focused beam onthe structure of interest T.

Optionally, the illumination system 312 provides a reference beam ofradiation to a reference detector 314 which may be configured to measurea spectrum and/or intensities of different wavelengths in the filteredbeam 342. The reference detector 314 may be configured to generate asignal 315 that is provided to processor 310 and the filter may compriseinformation about the spectrum of the filtered beam 342 and/or theintensities of the different wavelengths in the filtered beam.

Reflected radiation 360 is captured by detector 318 and a spectrum isprovided to processor 320 for use in calculating a property of thetarget structure T. The illumination system 312 and detection system 318thus form an inspection apparatus. This inspection apparatus maycomprise a soft X-ray and/or EUV spectroscopic reflectometer of the kinddescribed in US2016282282A1 which content is incorporated herein byreference in its entirety.

If the target T has a certain periodicity, the radiation of the focusedbeam 356 may be partially diffracted as well. The diffracted radiation397 follows another path at well-defined angles with respect to theangle of incidence then the reflected radiation 360. In FIG. 4, thedrawn diffracted radiation 397 is drawn in a schematic manner anddiffracted radiation 397 may follow many other paths than the drawnpaths. The inspection apparatus 302 may also comprise further detectionsystems 398 that detect and/or image at least a portion of thediffracted radiation 397. In FIG. 4 a single further detection system398 is drawn, but embodiments of the inspection apparatus 302 may alsocomprise more than one further detection system 398 that are arranged atdifferent position to detect and/or image diffracted radiation 397 at aplurality of diffraction directions. In other words, the (higher)diffraction orders of the focused radiation beam that impinges on thetarget T are detected and/or imaged by one or more further detectionsystems 398. The one or more detection systems 398 generates a signal399 that is provided to the metrology processor 320. The signal 399 mayinclude information of the diffracted light 397 and/or may includeimages obtained from the diffracted light 397.

To aid the alignment and focusing of the spot S with desired productstructures, inspection apparatus 302 may also provide auxiliary opticsusing auxiliary radiation under control of metrology processor 320.Metrology processor 320 can also communicate with a position controller372 which operates the translation stage, rotation and/or tiltingstages. Processor 320 receives highly accurate feedback on the positionand orientation of the substrate, via sensors. Sensors 374 may includeinterferometers, for example, which can give accuracy in the region ofpicometers. In the operation of the inspection apparatus 302, spectrumdata 382 captured by detection system 318 is delivered to metrologyprocessing unit 320.

As mentioned an alternative form of inspection apparatus uses soft X-rayand/or EUV radiation at normal incidence or near-normal incidence, forexample to perform diffraction-based measurements of asymmetry. Bothtypes of inspection apparatus could be provided in a hybrid metrologysystem. Performance parameters to be measured can include overlay (OVL),critical dimension (CD), coherent diffraction imaging (CDI) andat-resolution overlay (ARO) metrology. The soft X-ray and/or EUVradiation may for example have wavelengths less than 100 nm, for exampleusing radiation in the range 5-30 nm, of optionally in the range from 10nm to 20 nm. The radiation may be narrowband or broadband in character.The radiation may have discrete peaks in a specific wavelength band ormay have a more continuous character.

Like the optical scatterometer used in today's production facilities,the inspection apparatus 302 can be used to measure structures withinthe resist material treated within the litho cell (After DevelopInspection or ADI), and/or to measure structures after they have beenformed in harder material (After Etch Inspection or AEI). For example,substrates may be inspected using the inspection apparatus 302 afterthey have been processed by a developing apparatus, etching apparatus,annealing apparatus and/or other apparatus.

FIG. 5 illustrates an illumination source apparatus 500 according to afirst embodiment of the invention. The illumination source apparatus maybe used in a metrology apparatus, such as that described above withreference to FIG. 4. The illumination source apparatus comprises a highharmonic generation, HHG, medium 502 delivered by a gas nozzle 504. Apump radiation source 506 is operable to emit a beam of pump radiation508. The beam of pump radiation emitted by the pump radiation sourcetypically has a Gaussian transverse spatial profile with a beam waistdenoted w₀. Unlike conventional HHG sources where the Gaussian beam ofpump radiation is focused directly into the HHG medium for conversion,according to embodiments of the invention, the beam of pump radiation isfirst incident upon a set of adjustable transformation optics, showngenerally at 510 in FIG. 5. The purpose of the adjustable transformationoptics is to adjustably transform the transverse spatial profile of thebeam of pump radiation to produce a transformed beam having a differenttransverse spatial profile.

According to this embodiment, the adjustable transformation optics 510comprise a pair of refractive axicon elements 512, 514 of equal apexangle τ. The refractive axicon elements are placed in series with anaxial separation distance D₁. A refractive axicon is a conical opticalelement which may be either positive (convex) or negative (concave).Refractive axicon elements are characterized by an apex angle τ and therefractive index of the medium forming the axicon element. Theproperties of negative and positive refractive axicons are described indetail with reference to FIGS. 14a and 14b respectively. By “refractive”it is to be understood that the elements operate in transmission and areformed of a material having a refractive index generally greater than orless than that of the medium surrounding the axicon element. Withreference to FIG. 14a , the negative (concave) refractive axicon elementis characterized by an apex angle τ and is formed of a medium having arefractive index denoted n_(axicon) which may be in the range from 1.4to 1.6. The refractive index of the surrounding medium (e.g. air orvacuum) is denoted n and is typically 1.0. The angle at which incidentparallel rays of light/radiation are deflected away from the opticalaxis is denoted γ and is given by the expression:

$\begin{matrix}{\gamma = {\left( \frac{n_{axicon} - n}{n} \right){\left( \frac{\pi \pm \tau}{2} \right).}}} & (1)\end{matrix}$

With reference to FIG. 14b , the positive (convex) refractive axiconelement is also characterized by an apex angle τ and is also formed of amedium having a refractive index denoted n_(axicon) which may be in therange from 1.4 to 1.6. An equivalent deflection angle γ arises also forthe case of a positive axicon, as is apparent from FIG. 14b , and can beconsidered to be the angle at which an incident ray of light/radiationis deflected towards the optical axis. This is defined by the sameequation (1) above.

In the embodiment shown in FIG. 5, the first axicon element 512 of theadjustable transformation optics 510, relative to the direction ofpropagation of the beam of pump radiation, i.e. from left to right inFIG. 5, is a negative (concave) refractive axicon element. The effect ofthe first axicon element on the incident beam of pump radiation is tobend it away from the optical axis 516 which is also the centre axis ofthe beam of pump radiation and shown as the +z direction in FIG. 5.Since the first axicon element is on axis, the beam of pump radiationdiverges away from the optical axis as a hollow ring of light,increasing in radius linearly with distance along the positive zdirection. The second axicon element 514 is a positive (convex)refractive axicon element placed at an axial separation distance D₁ fromthe first axicon element. The second axicon element bends the beam backtowards the optical axis 516 (substantially) canceling out thedivergence imparted by the first axicon element, thereby resulting in acollimated annular beam 518 referred to herein as the “transformedbeam”. The divergence is canceled out since the first and second axiconelements have equal but opposite apex angles τ. Alternatively, it isenvisaged that the two axicon elements could have slightly differentapex angles thereby resulting in a non-collimated annular beam. In thiscase, the focusing element 520, discussed below, could be configured toat least partially compensate for the non-collimated nature of theannular beam. One or both of the axicons is mounted on a movable mountto allow the separation D₁ in the z direction, i.e. along the opticalaxis, to be adjusted.

A sample transverse intensity profile in the (x, y) plane is show inFIG. 6. The resulting annular beam 518 is characterized by the innerring radius R₁ and the outer ring radius R₂. The Gaussian tail from theoriginal pump beam remains on the outside of the annular beam, whilstthe inner edge of the annular beam is sharp. The transformed beam 518has radii R₁ and R₂ given by:

R₁=D₁ tan γ;   (2)

R ₂ =R ₁ +w _(o),   (3)

where the angle γ is the angle at which radiation is deflected awayfrom, or towards, the optical axis as defined above in equation (1) andin equation (5).

In this manner, since the transformation optics produce a transformedbeam resembling a collimated annular beam, relative to the centre axisof the transformed beam, a central region of the transformed beam hassubstantially zero intensity and an outer region—which is radiallyoutwards from the centre axis of the transformed beam—has a non-zerointensity. The location of the outer region which has non-zero intensityis dependent on an adjustment setting of the adjustable transformationoptics. For example, in this embodiment it is dependent on theseparation distance D₁ between the first and second axicons. Thisseparation distance is adjustable using the one or more movable mountsas described above.

Between the transformation optics and the HHG medium is a focusingelement 520 which focuses the transformed beam 518 into the HHG medium.The focusing element is located at a distance D₂ from the second axiconelement. The transformed beam is arranged to excite the HHG medium so asto generate high harmonic radiation, rather than exciting the HHG mediumwith the original Gaussian beam of pump radiation as is the case inconventional HHG sources. In this embodiment the focusing element is alens 520 having focal length f. The focal plane of the lens ispositioned substantially in the HHG medium. In this manner, since thetransformed beam before the lens is collimated, the transformed beam isfocused by the lens to a high intensity spot substantially in the HHGmedium where soft x-ray and/or EUV radiation is generated.

The portion of the transformed beam which is not converted to soft x-rayradiation and/or EUV in the HHG process propagates away from the focalspot and will reform the hollow annular intensity profile in the farfield. On the other hand, the generated high harmonic (soft x-ray and/orEUV) radiation, 540, will propagate substantially along the optical axisand not form a hollow annular beam. This is because at the focus, thetransformed beam has a similar field distribution to a Gaussian beam.Since the generated soft x-ray and/or EUV radiation and residualtransformed beam become spatially separated in the far field, a passiveblocking element 522 is positioned after the HHG medium and is used toblock/suppress the residual transformed beam remaining after HHG, whilstsubstantially transmitting the generated soft x-ray and/or EUV radiationfor use in e.g. a metrology apparatus. In this embodiment, the blockingelement is an output aperture 522 aligned with the centre axis of thegenerated high harmonic radiation. The distance between the focal planeof the lens and the output aperture is D₃. Any remaining pump radiationfrom the transformed beam after the output aperture may optionally befiltered out by a Zirconium filter 524.

The location of the first axicon element 512 is chosen such that it isin the conjugate image plane of the output aperture 522. This imagingrelationship has proven to be optimal for suppression of the residualtransformed beam by means of the output aperture. According to the thinlens formula, the relation between the distances in FIG. 5 is thus:

$\begin{matrix}{{\frac{1}{\left( {D_{1} + D_{2}} \right)} + \frac{1}{\left( {f + D_{3}} \right)}} = \frac{1}{f}} & (4)\end{matrix}$

FIG. 7 illustrates an illumination source apparatus 700 according to asecond embodiment of the invention. This embodiment is identical to thefirst embodiment shown in FIG. 5 except that according to thisembodiment an additional optical element in the form of an inputaperture 526 is provided and forms part of the transformation optics,shown generally at 510′. The input aperture is located on the centreaxis of the beam of pump radiation 516 and is positioned after the firstand second axicon elements and prior to the HHG medium with respect tothe direction of propagation of the beam of pump radiation. In thisembodiment, the lens 520 is configured to image the input aperture 526onto the blocking element 522, compared to the first embodiment wherethe lens is configured to image the first axicon element onto theblocking element. In use, the aperture size of the input aperture isselected in order to enable control over the outer ring radius R₂. Thisenables the inner and outer ring radii, R₁ and R₂ respectively, to beindependently tuned by adjusting the axicon separation D₁ and the inputaperture size to control R₂.

FIG. 8 illustrates an illumination source apparatus 800 according to athird embodiment of the invention. This embodiment is identical to thefirst embodiment shown in FIG. 5 except that it additionally includes avariable beam expander/contractor 528 before the pair of axiconelements. The variable beam expander/contractor thus forms part of thetransformation optics, shown generally at 510″. The variable beamexpander/contractor could be a telescope, for example. The variable beamexpander/contractor 528 enables the waist size w₀ of the beam of pumpradiation to be adjusted prior to manipulation by the pair of axiconelements. This enables the inner and outer ring radii, R₁ and R₂respectively, to be independently tuned by adjusting the axiconseparation D₁ and the input pump beam waist w₀, as can be seen from therelations shown in equations (2) and (3).

FIG. 9 illustrates the transformation optics part 510′″ of anillumination source apparatus according to a fourth embodiment of theinvention. This embodiment uses a pair of reflective axicon elements902, 904 instead of a pair of refractive axicon elements as describedabove with reference to the first three embodiments of the invention.According to this embodiment, the first axicon element 902 is a negative(convex) reflective axicon which is arranged with its tip on the centreaxis 906 of the beam of pump radiation and which is configured toreflect the beam of pump radiation towards the second axicon element 904which is an annular reflective concave axicon configured to collimatethe beam to thereby produce said transformed beam. Since the secondaxicon element 904 is annular, the input beam of pump radiation can passthrough the centre of the second axicon element in order to reach thefirst axicon element.

The axicon elements 902 and 904 have equal apex angles τ. The firstaxicon element 902 is shown in more detail in FIG. 10 a. The firstaxicon element 902 has an apex angle τ and is rotationally symmetricaround the tip of the cone. The tip of the cone is centred along theoptical axis 906. In this manner, the beam of pump radiation whichpropagates along the same optical axis in the +z direction is reflectedat a fixed and constant angle γ away from the optical axis in thedirection opposite to the incident propagation direction, i.e. in the −zdirection, where:

$\begin{matrix}{\gamma = {\frac{\left( {\pi \pm \tau} \right)}{2}.}} & (5)\end{matrix}$

The reflected beam will form a diverging hollow annular beam, with aninner radius R₁ increasing with distance D₁ from the first axiconelement according to equation (2). The radial ring width of the annularbeam is half the incident beam diameter, or in the case that the beam ofpump radiation is a collimated Gaussian beam, this is the same as thepump beam waist w₀. The outer ring radius R₂ is given by equation (3).The second axicon reflector element 904 is a positive (concave)reflective axicon and has the same apex angle τ as the first element,and is shown in more detail in FIG. 10 b. This element is an annularmirror with a circularly symmetric fixed-angle surface. The centralregion of the element is hollow to allow the unhindered transmission ofthe initial beam of pump radiation into the transformation optics 510′″,as shown in the plan view of FIG. 10 c. The annular reflector is used tocollect the light reflected from the first axicon element. The angle ofthe reflective surface is such that is corrects for the diverging cone,forming a collimated annular beam. After the second axicon element, anadditional optical element in the form of an input aperture 908 isprovided. The input aperture is located on the centre axis of the beamof pump radiation and is positioned after the first and second axiconelements and prior to the HHG medium with respect to the direction ofpropagation of the beam of pump radiation. In this embodiment, the lens(not shown) is configured to image the input aperture 908 onto theblocking element. The arrangement after the transformation optics 510′″is structurally the same as that shown in FIGS. 5, 7 and 8. In use, theaperture size of the input aperture is selected in order to enablecontrol over the outer ring radius R₂. This enables the inner and outerring radii, R₁ and R₂ respectively, to be independently tuned byadjusting the axicon separation D₁ and the input aperture size tocontrol R₂. Alternatively, this embodiment may use a variable beamexpander/contactor instead of or in addition to the input aperture 908.The variable beam expander/contractor would function in the mannerdescribed above with reference to the third embodiment. Alternatively,this embodiment, and all other embodiments, may not include a variablebeam expander/contractor or input aperture, if sufficient control overthe input beam waist w₀ is possible directly from the source of pumpradiation.

FIG. 15 illustrates the transformation optics part 510″″ of anillumination source apparatus according to a fifth embodiment of theinvention. This embodiment is similar to the fourth embodiment describedwith reference to FIG. 9 because it also uses a pair of reflectiveaxicon elements 1002 and 1004 in the transformation optics, as opposedto refractive axicon elements. According to this embodiment, the firstaxicon element 1002 is a negative (convex) reflective axicon which isarranged with its tip on the centre axis 1006 of the beam of pumpradiation but angled at 45 degrees to the optical axis. In this mannerthe first axicon element 1002 reflects the beam of pump radiationtowards the second axicon element 1004 which is a positive (concave)reflective axicon configured to collimate the beam to thereby producesaid transformed beam. The second axicon element is also angled at 45degrees to the optical axis 1006 and is parallel to the first axiconelement. The axicon elements 1002 and 1004 have equal apex angles r.Unlike the fourth embodiment, in this embodiment the second axiconelement does not have an annular geometry since the input beam of pumpradiation does not pass through the second axicon element in order toreach the first axicon element. In many aspects this embodimentfunctions in the same manner as the fourth embodiment, and equation (5)defines the deflection angle γ and equations (2) and (3) determine theannular beam radii in dependence on γ, the axicon separation D₁, andinput pump beam waist w₀.

FIG. 16 illustrates the transformation optics part 510′″″ of anillumination source apparatus according to a sixth embodiment of theinvention. This embodiment uses a pair of diffractive optical element(DOE) axicons 161 and 162 in the transformation optics, as opposed torefractive and reflective axicon elements. According to this embodiment,the first diffractive axicon element 161 is a negative (convex) DOEwhich is arranged on the centre axis 160 of the beam of pump radiation.In this manner the first DOE 161 diffracts the beam of pump radiationtowards the second DOE axicon 162 which is a positive (concave) DOEconfigured to collimate the beam to thereby produce said transformedbeam. The diffractive axicon elements 161 and 162 have equal butopposite divergence angle β. In many aspects this embodiment functionsin the same manner as the first embodiment, and equation (7) defines thedivergence angle β and equations (7), (8) and (3) determine the annularbeam radii in dependence on γ, the axicon separation D₁, and input pumpbeam waist w₀.

In all of the above embodiments, the axial separation of the axicons,D₁, is selected by means of the one or more movable mounts on which theyare mounted in order to optimize certain parameters of the illuminationsource, as explained in more detail below. In addition, the input waistsize, w₀, of the beam of pump radiation may be adjusted (e.g. using thevariable beam expander/contractor) in order to further optimize saidparameters. For the embodiments also including an input aperture as partof the transformation optics, the aperture size of the input aperturemay be adjusted to further optimize said parameters.

A first parameter of the illumination source to be optimized is theconversion efficiency of the high harmonic generation process. Optimumsoft X-ray and/or EUV generation is achieved in HHG by having the peakintensity region of the pump radiation inside or near the HHG medium,and by matching the phase of the pump radiation to that of the generatedsoft X-ray and/or EUV radiation as best as possible. The HHG medium islocated substantially in the focal plane of the lens. The transformationoptics comprise a negative and positive axicon pair (each having thesame apex angle τ) to produce a collimated annular beam. As such thepeak intensity of the pump radiation will be within the HHG medium.Furthermore, embodiments of the invention also enable the phase velocityof the pump radiation at the focus within the HHG medium to betuned—thereby enabling the phase matching and hence conversionefficiency to be improved. To understand this effect, the Gouy phasethrough the focus of the transformed beam of pump radiation having anannular transverse spatial profile is approximately given by thefollowing expression:

$\begin{matrix}{\phi_{Gouy} = {{z\left( {k - \left( {k*{\cos \left( {\arctan \left( \frac{R_{av}}{f} \right)} \right)}} \right)} \right)} + {\arctan \left( \frac{{zR}_{av}k\; \Delta \; R}{{\pi 2}{.8}f^{2}} \right)}}} & (6)\end{matrix}$

where R_(av)=(R₁+R₂)/2, ΔR=R₂−R₁, f is the focal length of the focusingelement, k is the wave vector and z is the axial position relative tothe focus at z=0. Since R₁ and R₂ are independently selectable byadjusting the axicon separation, D₁, and the input beam waist w₀, or theaperture size of the input aperture in those embodiments using an inputaperture, the Gouy phase of the transformed beam through the focus inthe HHG medium may be controllably influenced—thereby controlling thephase matching between the transformed beam and the generated highharmonic radiation. FIG. 11 shows the results of numerical modeling ofthe phase velocity through the focus of various annular beams ofincreasing inner radius R₁ and having a focus at z=0. The bottom line inthe plot of FIG. 11 corresponds to a Gaussian beam (i.e. R₁=0 mm) withthe subsequent lines corresponding to increasing R₁ radius up to amaximum value of R₁=7 mm for the top line, i.e. the annular beam withthe greatest phase velocity. FIG. 11 therefore clearly demonstrates thatthe phase velocity of the transformed pump beam around the focusincreases as R₁ increases. It is therefore possible to attain a targetphase velocity by choosing appropriate values for R₁ and R₂ within theconstraints imposed by other considerations, such as for example thesuppression of the residual transformed beam, which is discussed below.

A second parameter of the illumination source to be optimized inembodiments of the invention, either in addition to or separate from thefirst parameter above, is the suppression of the residual transformedbeam remaining after the high harmonic generation process. Away from thefocal plane at z=0, the residual transformed pump beam diverges into ahollow annular beam again, as described above with reference to FIGS. 5,7 and 8. The blocking element in the form of an output aperturepositioned in the far-field is matched to the width of the annulus ofthe transformed beam to enable removal of the residual transformed beam.This relies upon an emission cone of the generated HHG radiation that issmaller in transverse extent than that of the transformed beamdivergence such that the output aperture will solely interact with theresidual transformed beam and not the generated HHG radiation.Simulation results indicate that a suppression of the residualtransformed beam of better than 10⁻⁵ is achievable if the divergence ofthe generated HHG radiation is less than 2.5 mrad. This suppressionvalue is for an ideal situation with no scatter effects due to imperfectoptical components. In one example, the output aperture may have adiameter of 2.5 mm and be placed a distance 500 mm from the focal plane.FIG. 12 shows the suppression of the residual transformed beam for thisuse case as a function of the input pump beam waist w₀ and the axiconpair separation D₁. The expected operating range for optimum soft x-rayand/or EUV emission is an axicon separation in the range 10 cm<D₁<25 cmand R₂ radius in the range 1.5 mm<R₂<3 mm.

In all of the above-described embodiments, the second axicon element andthe focusing element could be combined into a single optical element.Further, a positive axicon may be used in place of any negative axiconin each of the above embodiments. This is because whilst a positiveaxicon initially causes incident radiation to be deflected towards theoptical axis, after a sufficient propagation distance the radiation willpass through a ‘focus’ and begin to diverge away from the optical axisthe other side of the focus.

The term “positive” as used with reference to an axicon element (eitherrefractive, reflective or diffractive) should be understood to mean anaxicon element which causes incident radiation to be deflected towardsthe optical axis. The term “negative” as used with reference to anaxicon element (either refractive, reflective or diffractive) should beunderstood to mean an axicon element which causes incident radiation tobe deflected away from the optical axis. The angle γ is used herein todenote the deflection angle at which incident radiation is deflectedeither towards or away from the optical axis by the positive/negativeaxicon having apex angle τ. Expressions linking the physical apex angleof the axicon, τ, with the deflection angle γ are given in equations (1)and (5) for the cases of refractive and reflective axicons respectively.

In a further embodiment of the invention, instead of using an axiconpair as used in all of the above-described embodiments, alternatively avariable beam expander/contractor can be used in combination with acircular beam block which is configured to block the central region ofthe expanded/contracted beam of pump radiation. In this manner anannular beam is generated which can be used to excite the HHG medium togenerate soft x-ray and/or EUV radiation. The generated annular beamparameters can be adjusted to optimize the conversion efficiency and/orsuppression of the residual pump beam. However, compared to the aboveembodiments, in which a negligible amount of the original pump beamenergy is lost, a significant fraction of the original pump beam energyis lost due to absorption by the circular beam block.

In all of the above-described embodiments, the reflective or refractiveaxicon element may be replaced with diffractive optical elements (DOE).The DOE may replicate the refractive or reflective axicons and addfurther ability to modify the focal plane intensity and phase, forexample, adding the function of the phase step like the TOP mirror does.When light incidents on the DOE, the structures on DOE diffract theincident light into a pre-determined intensity and phase distribution.DOE elements may be classified into two types, phase and amplitude orboth. Optionally, an axicon DOE may be an annular grating of even radialperiod.

The first negative diffractive axicon element diffract the beam awayfrom the optical axis forming a diverging annular beam. The secondpositive diffractive axicon corrects the diverging annulus forming acollimated annular beam. The DOE axicon is defined not by an apex anglebut a divergence angle β. The divergence angle β is equivalent to twiceas the deflection angle (2*Y). The divergence angle is given by thestandard grating equation, where λ is the wavelength and Λ is thediffraction period:

$\begin{matrix}{\beta = {2{{\sin^{- 1}\left( \frac{\lambda}{\Lambda} \right)}.}}} & (7)\end{matrix}$

The annulus radius R₁ as can be seen in FIG. 6, at a given distance fromthe element D₁ is given by:

$\begin{matrix}{R_{1} = {D_{1} \cdot {{\tan \left( \frac{\beta}{2} \right)}.}}} & (8)\end{matrix}$

In a further embodiment of the invention, two DOE elements of oppositedivergence angle β are placed in series, a collimated annular beam isformed.

A further use of using DOE is the ability to customize the grating tointroduce extra phase modulations in order to generate a custom focalplane intensity and phase profile.

In a further embodiment of the invention, more than one DOEs arecombined in parallel or in series way in a single illumination sourceapparatus.

Spatial light modulators (SLM) can be used to generate fully customprogrammable DOE which can be used to generated diffraction gratings. AnSLM allows for real time modifications of the beam, which can be eitherused in conjunction with a fixed form DOE or as a design input for afixed form DOE.

FIG. 13 illustrates a method of operating an illumination sourceapparatus, suitable for use in a metrology apparatus for thecharacterization of a structure on a substrate, according to anembodiment of the invention. The method comprises:

providing a high harmonic generation, HHG, medium (S1);

operating a pump radiation source to emit a beam of pump radiation (S2);and

transforming, by adjustable transformation optics, the transversespatial profile of the beam of pump radiation to produce a transformedbeam such that relative to the centre axis of the transformed beam, acentral region of the transformed beam has substantially zero intensityand an outer region which is radially outwards from the centre axis ofthe transformed beam has a non-zero intensity, wherein the transformedbeam excites the HHG medium so as to generate high harmonic radiation,(S3), wherein the location of said outer region is dependent on anadjustment setting of the adjustable transformation optics.

Further embodiments are disclosed in the subsequent numbered clauses:

-   1. An illumination source apparatus, suitable for use in a metrology    apparatus for the characterization of a structure on a substrate,    the illumination source apparatus comprising:

a high harmonic generation, HHG, medium;

a pump radiation source operable to emit a beam of pump radiation; and

adjustable transformation optics configured to adjustably transform thetransverse spatial profile of the beam of pump radiation to produce atransformed beam such that relative to the centre axis of thetransformed beam, a central region of the transformed beam hassubstantially zero intensity and an outer region which is radiallyoutwards from the centre axis of the transformed beam has a non-zerointensity, wherein the transformed beam is arranged to excite the HHGmedium so as to generate high harmonic radiation,

wherein the location of said outer region is dependent on an adjustmentsetting of the adjustable transformation optics.

-   2. The illumination source apparatus according to clause 1, wherein    the pump radiation source is operable to emit a beam of pump    radiation with a Gaussian transverse spatial profile, and wherein    the adjustable transformation optics are configured to produce a    transformed beam with a non-Gaussian transverse spatial profile.-   3. The illumination source apparatus according to clause 1, wherein    the pump radiation source is operable to emit a beam of pump    radiation with a Gaussian transverse spatial profile, and wherein    the adjustable transformation optics are configured to produce a    transformed beam having an annular transverse spatial profile.-   4. The illumination source apparatus according to any preceding    clause, further comprising a focusing element positioned between the    adjustable transformation optics and the HHG medium, the focusing    element configured to focus the transformed beam into the HHG    medium.-   5. The illumination source apparatus according to clause 4, wherein    the focusing element is a lens.-   6. The illumination source apparatus according to clause 4 or 5,    wherein the focal plane of the focusing element is positioned    substantially in the HHG medium.-   7. The illumination source apparatus according to any one of clauses    4 to 6, wherein the adjustable transformation optics comprise at    least one conical optical element or diffractive optical element.-   8. The illumination source apparatus according to clause 7, wherein    the at least one diffractive optical element or conical optical    element is an axicon element.-   9. The illumination source apparatus according to any preceding    claim, wherein the adjustable transformation optics comprise a pair    of axicon elements consisting of a first axicon element and a second    axicon element, wherein the first axicon element precedes the second    axicon element relative to the propagation direction of the beam of    pump radiation and wherein an axial separation between the first    axicon element and the second axicon element controls said    adjustment setting of the adjustable transformation optics.-   10. The illumination source apparatus according to clause 9, wherein    the pair of axicon elements consists of one negative axicon element    and one positive axicon element.-   11. The illumination source apparatus according to clause 9 or 10,    wherein at least one of the axicon elements is a reflective axicon    element.-   12. The illumination source apparatus according to clause 10 or 11,    wherein the first axicon element is a negative reflective axicon    which is arranged on the centre axis of the beam of pump radiation    and which is configured to reflect the beam of pump radiation    towards the second axicon element which is an annular positive    reflective axicon configured to collimate the beam to thereby    produce said transformed beam.-   13. The illumination source apparatus according to clause 9 or 10,    wherein at least one of the axicon elements is a refractive axicon    element.-   14. The illumination source apparatus according to clause 13,    wherein the first axicon element is a negative refractive axicon    which is arranged on the centre axis of the beam of pump radiation    and configured to diverge the beam of pump radiation towards the    second axicon element which is a positive refractive axicon arranged    on said centre axis and configured to collimate the beam to thereby    produce said transformed beam.-   15. The illumination source apparatus according to clause 9 or 10,    wherein at least one of the axicon elements is a diffractive    element.-   16. The illumination source apparatus according to clause 15,    wherein the first axicon element is a negative diffractive axicon    which is arranged on the centre axis of the beam of pump radiation    and configured to diverge the beam of pump radiation towards the    second axicon element which is a positive diffractive axicon    arranged on said centre axis and configured to collimate the beam to    thereby produce said transformed beam.-   17. The illumination source apparatus according to any one of    clauses 9 to 16, wherein each axicon of the pair of axicon elements    has substantially the same apex angle, τ, or the same divergence    angle, β, and the axicon elements are mounted on one or more movable    mounts such that said axial separation, D₁, between the axicon    elements is adjustable in use to control said adjustment setting.-   18. The illumination source apparatus according to clause 17,    further comprising a blocking element positioned after the HHG    medium, the blocking element configured to suppress the residual    transformed beam remaining after high harmonic generation, whilst    substantially transmitting the generated high harmonic radiation.-   19. The illumination source apparatus according to clause 18,    wherein the blocking element is an output aperture aligned with the    centre axis of the generated high harmonic radiation.-   20. The illumination source apparatus according to clause 18 or 19,    wherein the focusing element is configured to image the first axicon    element onto the blocking element.-   21. The illumination source apparatus according to any one of    clauses 18 to 20, wherein in use the axial separation of the    axicons, D₁, is selected by means of the one or more movable mounts    in order to optimize, for a given axicon apex angle τ or a given    divergence angle β:

(A) the conversion efficiency of the high harmonic generation process;and/or

(B) the suppression of the residual transformed beam.

-   22. The illumination source apparatus according to clause 21,    wherein the adjustable transformation optics further comprise a    variable beam expander/contractor configured to adjust the input    waist size, w₀, of the beam of pump radiation, and wherein in use w₀    is selected in order to further optimize (A) and (B).-   23. The illumination source apparatus according to clause 22,    wherein the transformed beam is a collimated annular beam having an    annulus radius R₁ and a ring width R₂, wherein:

R ₁ =L tan(γ); and

R ₂ =R ₁ +w ₀,

where γ is the deflection angle (as defined herein) of the axicon.

-   24. The illumination source apparatus according to clause 23,    wherein the adjustable transformation optics further comprise an    input aperture located on the centre axis of the beam of pump    radiation, wherein the input aperture is positioned after the axicon    elements and prior to the HHG medium with respect to the direction    of propagation of the beam of pump radiation, and the focusing    element is configured to image the input aperture onto the blocking    element, and wherein in use the aperture size of the input aperture    is selected in order to further optimize (A) and (B).-   25. The illumination source apparatus according to clause 24,    wherein the input aperture is configured to adjust the ring width    R₂.-   26. A method of operating an illumination source apparatus, suitable    for use in a metrology apparatus for the characterization of a    structure on a substrate, the method comprising:

providing a high harmonic generation, HHG, medium;

operating a pump radiation source to emit a beam of pump radiation; and

transforming, by adjustable transformation optics, the transversespatial profile of the beam of pump radiation to produce a transformedbeam such that relative to the centre axis of the transformed beam, acentral region of the transformed beam has substantially zero intensityand an outer region which is radially outwards from the centre axis ofthe transformed beam has a non-zero intensity, wherein the transformedbeam excites the HHG medium so as to generate high harmonic radiation,

wherein the location of said outer region is dependent on an adjustmentsetting of the adjustable transformation optics.

-   27. The method according to clause 26, wherein the pump radiation    source emits a beam of pump radiation with a Gaussian transverse    spatial profile, and wherein the adjustable transformation optics    produce a transformed beam with a non-Gaussian transverse spatial    profile.-   28. The method according to clause 26, wherein the pump radiation    source emits a beam of pump radiation with a Gaussian transverse    spatial profile, and wherein the adjustable transformation optics    produce a transformed beam having an annular transverse spatial    profile.-   29. The method according to any one of clauses 26 to 28, further    comprising focusing the transformed beam into the HHG medium using a    focusing element positioned between the adjustable transformation    optics and the HHG medium.-   30. The method according to any one of clauses 26 to 29, wherein the    adjustable transformation optics comprise a pair of axicon elements    consisting of a first axicon element and a second axicon element,    wherein the first axicon element precedes the second axicon element    relative to the propagation direction of the beam of pump radiation    and wherein the axial separation between the first axicon element    and the second axicon element is adjusted to control said adjustment    setting of the adjustable transformation optics.-   31. The method according to clause 30, wherein each axicon of the    pair of axicon elements has substantially the same apex angle, τ, or    the same divergence angle, β, and the axicon elements are mounted on    one or more movable mounts, the method further comprising adjusting    the axial separation, D₁, between the axicon elements to control    said adjustment setting.-   32. The method according to clause 31, further comprising    suppressing, using a blocking element positioned after the HHG    medium, the residual transformed beam remaining after high harmonic    generation, whilst substantially transmitting the generated high    harmonic radiation.-   33. The method according to clause 32, further comprising imaging    the first axicon element onto the blocking element.-   34. The method according to any one of clauses 31 to 33, further    comprising selecting the axial separation, D₁, between the axicon    elements in order to optimize:

(A) the conversion efficiency of the high harmonic generation process;and/or

(B) the suppression of the residual transformed beam.

-   35. The method according to clause 34, further comprising adjusting    the input waist size, w₀, of the beam of pump radiation using a    variable beam expander/contractor, in order to further optimize (A)    and (B).-   36. The method according to clause 35, wherein the adjustable    transformation optics further comprise an input iris located on the    centre axis of the beam of pump radiation, wherein the input iris is    positioned after the axicon elements and prior to the HHG medium    with respect to the direction of propagation of the beam of pump    radiation, and the focusing element is configured to image the input    iris onto the blocking element, and wherein the aperture size of the    input iris is selected in order to further optimize (A) and (B).-   37. A computer program comprising instructions which, when executed    on at least one processor, cause the at least one processor to    control an apparatus to carry out a method according to any one of    clauses 26 to 36.-   38. A carrier containing the computer program according to clause    37, wherein the carrier is one of an electronic signal, optical    signal, radio signal, or non-transitory computer readable storage    medium.-   39. A lithographic apparatus comprising the illumination source    apparatus according to any one of clauses 1 to 25.-   40. A lithographic cell comprising the lithographic apparatus    according to clause 39.-   41. A metrology apparatus comprising an illumination source    apparatus according to any one of the clause 1 to 25.-   42. A lithographic cell comprising a metrology apparatus according    to clause 41.

In the context of the above document the term HHG or HHG source isintroduced. HHG refers to High Harmonic Generation or sometimes referredto as high order harmonic generation. HHG is a non-linear process inwhich a target, for example a gas, a plasma or a solid sample, isilluminated by an intensive laser pulse. Subsequently, the target mayemit radiation with a frequency that is a multiple of the frequency ofthe radiation of the laser pulse. Such frequency, that is a multiple, iscalled a harmonic of the radiation of the laser pulse. One may definethat the generated HHG radiation is a harmonic above the fifth harmonicand these harmonics are termed high harmonics. The physical process thatforms a basis of the HHG process is different from the physical processthat relates to generating radiation of the lower harmonics, typicallythe 2nd to 5th harmonic. The generation of radiation of the lowerharmonic relates to perturbation theory. The trajectory of the (bound)electron of an atom in the target is substantially determined by theCoulomb potential of the host ion. In HHG, the trajectory of theelectron that contributes to the HHG process is substantially determinedby the electric field of the incoming laser light. In the so-called“three step model” describing HHG, electrons tunnel through the Coulombbarrier which is at that moment substantially suppressed by the laserfield (step 1), follow a trajectory determined by the laser field (step2) and recombine with a certain probability while releasing theirkinetic energy plus the ionization energy in the form of radiation (step3). Another way of phrasing a difference between HHG and the generationof radiation of the lower harmonic is to define that all radiation withphoton energy above the ionization energy of the target atoms as “HighHarmonic” radiation, e.g. HHG generated radiation, and all radiationwith photon energy below the ionization energy as non-HHG generatedradiation. If Neon is used as a gas target, all radiation with awavelength shorter than 62 nm (having a photon energy higher than 20.18eV) is generated by means of the HHG process. For Argon as a gas target,all radiation having a photon energy higher than about 15.8 eV isgenerated by means of the HHG process.

Although specific reference is made in this text to “metrologyapparatus”, this term may also refer to an inspection apparatus or aninspection system, e.g. the inspection apparatus that comprises anembodiment of the invention may be used to detect defects of a substrateor defects of structures on a substrate. In such an embodiment, acharacteristic of interest of the structure on the substrate may relateto defects in the structure, the absence of a specific part of thestructure, or the presence of an unwanted structure on the substrate.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications. Possible other applications include the manufactureof integrated optical systems, guidance and detection patterns formagnetic domain memories, flat-panel displays, liquid-crystal displays(LCDs), thin-film magnetic heads, etc.

Although specific reference may be made in this text to embodiments ofthe invention in the context of a metrology apparatus, embodiments ofthe invention may be used in other apparatus. Embodiments of theinvention may form part of a mask inspection apparatus, a lithographicapparatus, or any apparatus that measures or processes an object such asa wafer (or other substrate) or mask (or other patterning device). Theseapparatuses may be generally referred to as lithographic tools. Such alithographic tool may use vacuum conditions or ambient (non-vacuum)conditions.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention, where the context allows, is notlimited to optical lithography and may be used in other applications,for example imprint lithography.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims set out below.

Although it has been mentioned that an axicon element is a reflectiveaxicon element or a refractive axicon element, it should be understoodthat the reflective axicon element or refractive axicon element may bereplaced with a diffractive axicon element.

1-15. (canceled)
 16. An illumination source apparatus, suitable for usein a metrology apparatus for the characterization of a structure on asubstrate, the illumination source apparatus comprising: a high harmonicgeneration, HHG, medium; a pump radiation source operable to emit a beamof pump radiation; and transformation optics configured to transform atransverse spatial profile of the beam of pump radiation to produce atransformed beam such that relative to a center axis of the transformedbeam, a central region of the transformed beam has substantially zerointensity, and an outer region which is radially outwards from thecenter axis of the transformed beam has a non-zero intensity, whereinthe transformed beam is arranged to excite the HHG medium so as togenerate high harmonic radiation, wherein the transformation opticscomprise a negative element.
 17. The illumination source apparatus ofclaim 16, wherein the transformation optics comprise a positive element.18. The illumination source apparatus of claim 17, wherein the positiveelement and the negative element have substantially a same apex angle,τ, or a same divergence angle, β.
 19. The illumination source apparatusof claim 17, wherein the positive element precedes the negative element,or the negative element precedes the positive element, relative topropagation direction of the beam of pump radiation.
 20. Theillumination source apparatus of claim 16, wherein the transformationoptics comprise a reflective element, a refractive element, or adiffractive element.
 21. The illumination source apparatus of claim 16,further comprising: a focusing element positioned between thetransformation optics and the HHG medium, the focusing elementconfigured to focus the transformed beam into the HHG medium, andwherein the focusing element is a lens.
 22. The illumination sourceapparatus of claim 21, wherein a focal plane of the focusing element ispositioned substantially in the HHG medium.
 23. The illumination sourceapparatus of claim 16, further comprising a blocking element positionedafter the HHG medium, the blocking element configured to suppressresidual transformed beam remaining after high harmonic generation,whilst substantially transmitting generated high harmonic radiation. 24.The illumination source apparatus of claim 23, wherein the blockingelement is an output aperture aligned with a center axis of thegenerated high harmonic radiation.
 25. The illumination source apparatusof claim 16, wherein the transformed beam is a collimated annular beamhaving an annulus radius R₁ and a ring width R₂, wherein:R ₁ =D ₁ tan(γ); andR ₂ =R ₁ +w ₀, where γ is deflection angle and w₀ is waist size of thebeam of pump radiation.
 26. The illumination source apparatus of claim16, wherein the transformation optics further comprise an input aperturelocated on the center axis of the beam of pump radiation.
 27. Theillumination source apparatus of claim 26, wherein the input aperture ispositioned with respect to the direction of propagation of the beam ofpump radiation.
 28. A method of operating an illumination sourceapparatus, suitable for use in a metrology apparatus for thecharacterization of a structure on a substrate, the method comprising:providing a high harmonic generation, HHG, medium; operating a pumpradiation source to emit a beam of pump radiation; and transforming, bytransformation optics, a transverse spatial profile of the beam of pumpradiation to produce a transformed beam such that relative to a centeraxis of the transformed beam, a central region of the transformed beamhas substantially zero intensity and an outer region which is radiallyoutwards from the center axis of the transformed beam has a non-zerointensity, wherein the transformed beam excites the HHG medium so as togenerate high harmonic radiation, wherein the transformation opticscomprise a negative element.
 29. A lithographic cell comprising theillumination source apparatus of claim
 16. 30. A metrology apparatuscomprising an illumination source apparatus of claim 16.